193707721

193707721 is the smallest prime factor of $$2^67-1$$.

Mersenne claimed the number 267 - 1 to be prime in 1876 without finding the factors of this number. Frank Nelson Cole walked to the board has discovered this prime factor of the 67th Mersenne number $$(147573952589676412927)$$. He has checked more than 10,000,000 primes to test the primality for this composite Mersenne number. After about 3 years of hard work, he wrote down this prime factor of 267 - 1 on a piece of paper and presented it on a board. Cole then multiplied 193707721 by 761838257287 by writing it down on a sheet of paper and got the result 147,573,952,589,676,412,927 which is equal to 267 - 1. Cole sat down for over an hour for doing this work of prime factorization. This Mersenne number was thought to be prime until 1903 until Cole found a 9-digit prime factor of this Mersenne number.